Spread-Spectrum refers to a broad class of modulation techniques in which a bandwidth of a carrier is enlarged beyond that necessary to convey the information to be transmitted. One technique for "spreading" the carrier signal is to replace each bit of the original message, with a determinate pseudo-random sequence of one's and zero's commonly known as a chipping sequence and then bi-phase or phase-reversal modulate the carrier signal in response to the sequence. As a result, a pattern of ten or possibly hundreds of bits of the chipping sequence may represent one or only a few bits of the original message. The sets of chipping sequences are generally mathematically orthogonal code patterns known by both the transmitter and receiver or generated by identical key generators seeded with the same predetermined value. The use of matched filters enables the receiver to enhance the signal upon reception thus allowing recovery of the signal-to-noise ratio (SNR) lost because of the increased bandwidth of the spread spectrum transmission.
The redundancy inherently present in spread spectrum modulation lowers channel information capacity, which tends to restrict useful applications due to its consumption of such large portions of the frequency spectrum while not conveying any more information. The information capacity limitation is further compounded in severely time dispersive channels.